In recent years, various techniques for crystallizing or improving the crystallinity of an amorphous or polycrystalline semiconductor film have been investigated. Such crystallized thin films may be used in the manufacture of a variety of devices, such as image sensors and active-matrix liquid-crystal display (“AMLCD”) devices. In the latter, a regular array of thin-film transistors (“TFTs”) is fabricated on an appropriate transparent substrate, and each transistor serves as a pixel controller.
Crystalline semiconductor films, such as silicon films, have been processed to provide pixels for liquid crystal displays using various laser processes including excimer laser annealing (“ELA”) and sequential lateral solidification (“SLS”) processes. SLS is well suited to process thin films for use in AMLCD devices, as well as organic light emitting diode (“OLED”) devices.
In ELA, a region of the film is irradiated by an excimer laser to partially melt the film, which subsequently crystallizes. The process typically uses a long, narrow beam shape that is continuously advanced over the substrate surface, so that the beam can potentially irradiate the entire semiconductor thin film in a single scan across the surface. The Si film is irradiated multiple times to create the random polycrystalline film with a uniform grain size. ELA produces small-grained polycrystalline films; however, the method often suffers from microstructural non-uniformities, which can be caused by pulse-to-pulse energy density fluctuations and/or non-uniform beam intensity profiles. FIG. 6A illustrates a random microstructure that may be obtained with ELA. This figure, and all subsequent figures, are not drawn to scale, and are intended to be illustrative in nature.
SLS is a pulsed-laser crystallization process that can produce high quality polycrystalline films having large and uniform grains on substrates, including substrates that are intolerant to heat such as glass and plastics. SLS uses controlled laser pulses to fully melt a region of an amorphous or polycrystalline thin film on a substrate. The melted regions of film then laterally crystallize into a solidified lateral columnar microstructure or a plurality of location-controlled large single crystal regions. Generally, the melt/crystallization process is sequentially repeated over the surface of a large thin film, with a large number of laser pulses. The processed film on substrate is then used to produce one large display, or even divided to produce multiple displays, each display being useful for providing visual output in a given device. FIGS. 6B-6D shows schematic drawings of TFTs fabricated within films having different microstructures that can be obtained with SLS. SLS processes are described in greater detail below.
The potential success of SLS systems and methods for commercial use is related to the throughput with which the desired microstructure and texture can be produced. The amount of energy and time it takes to produce a film having the microstructure is also related to the cost of producing that film; in general, the faster and more efficiently the film can be produced, the more films can be produced in a given period of time, enabling higher production and thus higher potential revenues.